Radiator Electric Fan Motor on a 2003 Buick Century won't come on when temp gets high

I have a 1997 Buick Century with the same problem. Both my fans work if the A/C is
on as they should. If your turn off the A/C the passanger side fan will not come on
and at an idle the coolant in the radiator will boil, you can hear it in the overflow tank.
If you are driving there is enough air flow thru the radiator that the coolant doesn't
boil and the car doesn't overheat. I have replace the water pump, the thermostat.
and the ETS or engine temperature sensor mounted just above the thermostat in
the thernostat housing, to no avail, I still have the same problem. The only other
thing is the coolant level sensor in the radiator, which I haven't replace yet due
to the fact you have to pull the radiator in order to replace it, which is a major
undertaking. I have a low coolant indicator on all the time on the dash so I have
a pretty good idea the coolanat level sensor is bad. The coolant level is not low
so that sensor is not working as it should. I hope this may help with your problem.
To test you fan raise the hood when it is cold and top off the radiator and fill the
overflow tank to the cold mark. Start the car and let it idle with the A/C off and
see if the drivers side fans comes on. It will have to get to operating temp
before the fan will come on. If it doesn't come on replace the radiator low coolant
sensor on the passanger side of the radiator. If that doesn't fix the problem
replace the thermostat and the Engine Temperature Sensor at the thermostat
housing with an A/C Delco Engine Temperature Sensor. Good Luck..